Magnesium (Mg) and its alloys provide numerous unique benefits as potential resorptive biomaterials and present the very real possibility of replacing current metallic implant materials in a variety of roles. However, considerable research remains before Mg alloys may be accurately screened and used in vivo. Most critically, a more comprehensive understanding of the corrosion of Mg alloys in vitro is needed. This research program critically examined the types of in vitro experiments that may be performed on Mg alloys, investigated the numerous variables that affect Mg biodegradation when undertaking these experiments, explored the electrochemical performance of several biocompatible Mg alloys, and developed a novel process for producing ordered Mg structures. The benefits and drawbacks of a range of in vitro tests were first investigated. The key strengths and weaknesses of each test were identified and recommendations provided for their respective use in the quest to determine Mg alloy biodegradation. The most common variables applicable to all in vitro experiments were then explored in detail, and their effect on the biocorrosion of a number of Mg alloys was determined. Recommendations were then made for the appropriate control of the different experimental variables based on these findings. For the first time, the mechanistic control of Mg biodegradation by the microstructure of biocompatible alloys has been examined. This allows for greater understanding of the reasons for varied corrosion of alloys in bio-electrolytes, and is a step towards the effective design of Mg alloys for different bio-applications. A novel method to produce ordered Mg structures was developed, with relevant processing parameters investigated in light of their effect on biocorrosion and mechanical performance. Overall, the results and findings from this research further our understanding of the potential of Mg alloys as suitable biomaterials, and advance our knowledge of how to proceed towards the goal of using such alloys for biological applications.

Studies concerning the formation of industrially useful polycarbonates are the focus of this dissertation. Of particular importance is the biodegradable polymer, poly(trimethylene carbonate) which has a wide range of medical applications. The production of polycarbonates can be achieved by the ring-opening polymerization of cyclic carbonate, or the copolymerization of carbon dioxide and oxiranes or oxetanes. For the production of polycarbonates from these monomers, Schiff base metal complexes have been designed, synthesized, and optimized as catalysts. Detailed kinetic and mechanistic studies have been performed for the ring-opening polymerization of cyclic carbonates, as well as the copolymerization of carbon dioxide and oxiranes or oxetane. In addition, the copolymerization of cyclic carbonates and cyclic esters to modify the mechanical and biodegradable properties of materials used for medical devices has been studied using biocompatible metal complexes. In the process for ring-opening polymerizations of trimethylene carbonate or lactides, Schiff base metal complexes (metal = Ca(II), Mg(II) and Zn(II)) have been shown to be very effective catalysts to produce high molecular weight polymers with narrow polydispersities. Kinetic studies demonstrated the polymerization reactions to proceed via a mechanism first order in [monomer], [catalyst], and [cocatalyst] if an external cocatalyst is applied, and to involve ring-opening by way of acyl-oxygen bond cleavage. The activation parameters (ΔH≠, ΔS≠ and ΔG≠) were determined for ringopening polymerization of trimethylene carbonate, ring-opening polymerization of lactides, and copolymerization of trimethylene carbonate and lactide. In the process for copolymerization of carbon dioxide and oxetane, metal salen derivatives of Cr(III) and Al(III) along with cocatalyst such as n-Bu4NX or PPNX (PPN = bis(triphenylphosphine)iminium, and X = Br, Cl and N3) have been shown to be effective catalysts to provide poly(trimethylene carbonate) with only trace amount of ether linkages. The formation of copolymer is proposed not to proceed via the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction. To support this conclusion, ring-opening polymerization of trimethylene carbonate has been performed and kinetic parameters have been compared with those from the copolymerization of carbon dioxide and oxetane.

PhD (Environmental Sciences) / Department of Ecology and Resource Management / The presence of arsenic in groundwater has drawn worldwide attention from researchers and public health officials due to its effects on human health such as, cancer, skin thickening, neurological disorders, muscular weakness, loss of appetite and nausea. World Health Organisation (WHO) has set the limit of 10 μg/L for arsenic in drinking water in trying to reduce the effects of arsenic. This was further adopted by South African National Standard (SANS). The present study aims at evaluating arsenic concentration in selected groundwater sources around Greater Giyani Municipality in Limpopo Province and further synthesize clay based adsorbents for arsenic removal using Fe3+ and Mn2+ oxides and hexadecylammonium bromide (HDTMA-Br) cationic surfactant as modifying agents. The first section of the work presented the hydrogeochemical characteristics of groundwater in the Greater Giyani Municipality. The results showed that the pH of the samples ranges from neutral to weakly alkaline. The dominance of major anionic and cationic species was found to be in the order: HCO3 ->Cl->SO4 2->NO3 - and Na+>Mg2+>Ca2+>K+>Si4+, respectively. Hydrogeochemical facies identified in the study area include CaHCO3 (90%) and mixed CaNaHCO3 (10%) which shows the dominance of water-rock interaction. About 60% of the tested samples contains arsenic concentration above 10 μg/L as recommended by SANS and WHO. Concentration of arsenic was found to be ranging between 0.1 to 172.53 μg/L with the average of 32.21 μg/L. In the second part of this work, arsenic removal efficiency of locally available smectite rich and kaolin clay was evaluated. Results showed that the percentage As(V) removal by kaolin clay was optimum at pH 2 while the percentage As(III) removal was greater than 60% at pH 2 to 12. For smectite rich clay soils, the percentage of As(III) and As(V) removal was found to be optimum at pH between 6 and 8. The adsorption isotherm data for As(III) and As(V) removal by both clays fitted better to Freundlich isotherm. Adsorption of both species of arsenic onto the clay mineral occurred via electrostatic attraction and ion exchange mechanisms. Both clay soils could be regenerated twice using Na2CO3 as a regenerant. Kaolin clay showed a better performance and was selected for further modification. In the third section of this work, Fe-Mn bimetal oxide modified kaolin clay was successfully synthesized by precipitating Fe3+ and Mn2+ metal oxides to the interlayer surface of kaolin clay. Modification of kaolin clay increased the surface area from 19.2 m2/g to 29.8 m2/g and further v decreased the pore diameter from 9.54 to 8.5 nm. The adsorption data fitted to the pseudo second order of reaction kinetics indicating that adsorption of As(III) and As(V) occurred via chemisorption. The adsorption isotherm data was described by Langmuir isotherm models showing a maximum As(III) and As(V) adsorption capacities of 2.16 and 1.56 mg/g, respectively at a temperature of 289 K. Synthesized adsorbent was successfully reused for 6 adsorptiondesorption cycles using K2SO4 as a regenerant. Column experiments showed that maximum breakthrough volume of ≈2 L could be treated after 6 hours using 5 g adsorbent dosage. Furthermore, the concentration of Fe and Mn were within the WHO permissible limit. In the fourth part of the work kaolin clay was functionalized with hexadecyltrimethylamonium bromide (HDTMA-Br) cationic surfactant and its application in arsenic removal from groundwater was investigated. The results revealed that adsorption of As(III) and As(V) is optimum at pH range 4-8. The maximum As(III) and As(V) adsorption capacities were found 2.33 and 2.88 mg/g, respectively after 60 min contact time. Pseudo first order model of reaction kinetics described the adsorption data for As(V) better while pseudo second order model described As(III) adsorption data. The adsorption isotherm data for As(III) and As(V) fitted well to Langmuir model indicating that adsorption of both species occurred on a mono-layered surface. Adsorption thermodynamics model revealed that adsorption of As(III) and As(V) was spontaneous and exothermic. The As(III)/As(V) adsorption mechanism was ascribed to electrostatic attraction and ion exchange. The regeneration study showed that synthesized adsorbent can be used for up to 5 times. In the firth part of the work inorgano-organo modified kaolin clay was successfully synthesized through intercalation of Fe3+ and Mn2+ metal oxides and HDTMA-Br surfactant onto the interlayers of the clay mineral. The batch experiments showed that As(III) removal was optimum at pH range of 4-6, while the As(V) removal was optimum at pH range 4-8. The adsorption data for both species of arsenic showed a better fit to pseudo second order of reaction kinetics which suggest that the dominant mechanism of adsorption was chemisorption. The isotherm studies showed better fit to Langmuir isotherm model as compared to Freundlich model. The maximum adsorption capacity As(III) and As(V) at room temperature as determined by Langmuir model were found to be 7.99 mg/g and 7.32 mg/g, respectively. The thermodynamic studies for sorption of As(III) and As(V) showed negative value of ΔGᴼ and ΔHᴼ indicating that adsorption process occurred spontaneously and is exothermic in nature. The regeneration study showed that the vi inorgano-organo modified kaolin clay can be reused for up 7 adsorption-regeneration cycles using 0.01 M HCl as a regenerant. Thomas kinetic model and Yoon-Nelson model showed that the rate of adsorption increases with increasing flow rate and initial concentration and decreases with increasing of the bed mass. In conclusions, adsorbents synthesized from this work showed a better performance as compared to other adsorbents available in the literature. Among the synthesized adsorbents, inorgano-organo modified clay showed highest adsorption capacity as compared to surfactant functionalized and Fe-Mn bimetal oxides modified kaolin clay. However, all adsorbents were recommended for use in arsenic remediation from groundwater. The following recommendations were made following the findings from this study: 1) routine monitoring of arsenic in groundwater of Greater Giyani Municipality, 2) evaluating the possible link between arsenic exposure and arsenic related diseases within Giyani in order to find the extent of the problem in order to establish the population at risk, 3) The toxicity assessment for HDTMA-Br modified kaolin clay should be carried out, 4) Materials developed in the present study should be modeled and tested at the point of use for arsenic removal, and lastly, 5) this study further encourage the development of other arsenic removal materials that can be used at household level. / NRF